Ignore empty passwd after repeated Enter keypress

Patch status: merged

Patch by Vytautas Šaltenis

Long description:

When no ignore_empty_password flag is specified, but Enter key is held
down for some time, i3lock enters a finite, but hours-long loop
verifying empty password. Don't do that, skip empty password
verification if nothing was pressed after last Enter keypress.
Again, making the software cat-proof.